1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
In a manufacturing process for a semiconductor device formed from an ultrafine pattern, such as an LSI and a VSLI, a reduction projection exposure apparatus is used, which performs reduction exposure of a circuit pattern drawn on a mask (reticle) onto a substrate (wafer) coated with a photosensitive agent to form a print pattern. With an increase in the packaging density of semiconductor devices, demands have arisen for further micropatterning. Along with the development of a resist process, exposure apparatuses have been increasingly required to cope with micropatterning.
As a means for increasing the resolving power of an exposure apparatus, there are available a method of changing an exposure wavelength to a shorter wavelength and a method of increasing the NA (Numerical Aperture) of a projection optical system. As the resolving power increases, the focal depth of the projection optical system decreases. It is therefore an important theme to improve the focus accuracy of matching the imaging plane (focal plane) of the projection optical system with a wafer surface.
In addition, one of the important optical characteristics of the projection exposure apparatus is the alignment accuracy of accurately superimposing the respective patterns throughout, a plurality of steps. Important factors that influence this alignment accuracy include a magnification error in the projection optical system. As the size of a pattern used for a VLSI has tended to decrease year by year, there have been increasing needs for an improvement in alignment accuracy. It is therefore very important to keep the magnification of the projection optical system at a predetermined value.
The projection optical system absorbs part of exposure energy. It is known that the temperature of the projection optical system changes due to the heat generated by the absorption, and the optical characteristics of the projection optical system, such as the refractive exponent, change. If the projection optical system is kept irradiated with exposure light for a long time, imaging characteristics of the projection optical system (for example, a focus, curvature of field, magnification, distortion, astigmatism aberration, and wavefront aberration) fluctuate. As a result, a line width error, alignment error, and the like may occur by an amount which cannot be neglected in terms of the device manufacture.
For this reason, there has been proposed a method of compensating for imaging characteristic fluctuations depending on the irradiated state of exposure energy onto the projection optical system. For example, according to Japanese Patent Publication No. 63-16725, the amount of fluctuation in imaging characteristic depending on the exposure energy state of a projection optical system is computed by a model formula using an exposure amount, an exposure time, a non-exposure time, and the like as variables, and the imaging characteristic fluctuation of the projection optical system is corrected based on the computation result. The above model formula has a coefficient unique to the projection optical system for each imaging characteristic. Properly setting this coefficient can obtain and correct the imaging characteristic fluctuation of the projection optical system.
In addition, there has been proposed an exposure apparatus which can obtain a higher resolving power with respect to the projection of a specific pattern by changing an illumination shape. In such an apparatus, the light source distribution generated on the pupil plane of a projection optical system changes depending on exposure conditions (for example, a projection system NA, the numerical aperture of an illumination system, an exposure area, an exposure center position, and a mask used for exposure), and hence the amounts of fluctuation in imaging characteristics occurring for the respective exposure conditions differ from each other.
Therefore, in order to accurately correct fluctuations in imaging characteristics depending on the above light source distribution state of illumination light, it is necessary to calculate correction coefficients optimal for exposure conditions from differences in the light source distribution state of illumination light on the pupil plane, reticle transmittance, exposure area, scanning speed, exposure amount, irradiation time, and the like.
However, correction coefficients differ for the respective exposure conditions, and it requires much time to grasp a thermal fluctuation phenomenon accompanying exposure. Japanese Patent Laid-Open No. 2002-15997 discloses a method of predicting the imaging characteristic fluctuation caused by lens heating, by using, for example, the following model formula:F(t)=A1(1−e−t/τ1)+A2((1−e−t/τ2  (1)where τ1 and τ2 are time constants, and A1 and A2 are amplitudes.
In addition, the amplitudes A1 and A2 are expressed as follows, assuming linear dependencies on some of the parameters indicating the amplitudes, especially parameters proportional to power entering a lens, such as a light intensity, the size of an imaging plane, a reticle transmittance, and a wafer reflectance.A1=μ1·Tr·S·I·Wref1  (2)A2=μ2·Tr·S·I·Wref1  (3)where I is an exposure intensity (W/m2), S is the size of an imaging plane at the wafer level or a mask area (m2), Tr is a reticle transmittance (a pure fraction or percentage), Wref1 is a wafer reflectance (a pure fraction or percentage), and μ1 and μ2 are correction coefficients. As described above, as a model formula for predicting an imaging characteristic fluctuation, a model formula is known to be obtained by modeling assuming a linear dependency on a parameter proportional to power entering the lens.
However, as in the case of the model formula described above, if the dependencies of the amplitudes A1 and A2 on an exposure angle of view are assumed in a linear form in the same manner, it is not possible to properly predict an imaging characteristic in a time zone immediately after the activation of the apparatus or a change in exposure condition, in which a fluctuation is relatively large.
In consideration of the above point, the present invention provides an exposure technique capable of predicting a change in imaging characteristic of the projection optical system upon a change in exposure angle of view and correcting the imaging characteristic.